This invention relates to the manufacture of bottles or containers of thermoplastic materials for the retention of fluids under pressure, such as carbonated beverages or the like. Recently, various thermoplastic materials have been developed which are capable of retaining carbon dioxide and which are blow-moldable into suitable containers. Such barrier materials include poly(ethyleneterephthalate) (PET), polyvinylchloride (PVC), or nitrile based resins known as LOPAC, a registered trademark of Monsanto Company, or nitrile-group-containing monomers of the type disclosed in U.S. Pat. No. 3,873,600.
One primary problem which is encountered in blow-molding thermoplastic materials to form bottles capable of retaining CO.sub.2 and other gases under pressure resides in the provision of a bottom shape capable of serving as a bottle support, while resisting deformation under pressure. Mere duplication in plastic of traditional glass bottom configurations is unsatisfactory because of the tendency of plastics to creep or become distorted under pressure, especially in the presence of the elevated temperatures which may be encountered during shipment and storage. Such distortion may alter the shape and dimensions of traditional bottom configurations to the extent that the level of liquid within the bottle falls below the fill line, thereby threatening customer acceptance or satisfaction, and the bottle may become a so-called rocker; that is, it may become unstable on a horizontal surface.
On the other hand, it is frequently desirable that the inner and outer shapes and dimensions of plastic bottles approximate those of glass bottles of the same capacity so that they may be handled by existing equipment and, in certain instances, assist customer identification of the particular product they contain. In any event, they should be aesthetically attractive.
A plastic bottle, when filled with a carbonated beverage and capped, must be able to withstand both the impact of falling from at least a moderate height onto a hard surface and the precipitous rise in internal pressure which accompanies the impact. While this requirement also affects selection of materials and bottle-forming techniques, it is an extremely important consideration in the contemplation of bottom design.
Finally, the optimum bottom structure is one which not only meets the foregoing criteria but which may be readily formed with an economy of material, without unduly expensive or elaborate equipment, and without intricate or additional manufacturing steps.
It has been known for a long time that the strongest pressure vessel is one of a spherical shape. Its excellent performance is based upon a uniform distribution of stresses, tensile stresses in case of internal pressure and compressive stresses in case of external pressure. However, spherical containers or those with hemispherical bottoms require extra stands or footing (base cups).
Next to the hemispherical there is the domed bottom which has a built-in upsidedown hemisphere, a cone or construction between the two. However, under pressure domed bottoms bulge also. Despite its rigidity, the dome loses some of its depth and the base diameter decreases. After these changes, the bottle becomes taller and not as stable.
One of the most common bottom structures employed in glass bottles of the type intended to contain beverages under pressure is the so-called "champagne bottom," the outer surface of which comprises a central concavity and a convex heel surrounding the concavity and merging therewith and with an end portion of the container sidewall. The lowermost points of the heel lie in a common plane to support the bottle in an upright position on a horizontal surface. Such a bottom configuration in the appropriate thickness may be wholly satisfactory in glass because of the rigidity of that material.
When the champagne bottom is translated to a thin-walled plastic container, however, the central concavity has a tendency to evert to convexity under internal pressure, thereby rendering the bottle unstable on a horizontal surface. Even if outright eversion does not occur, internal pressure tends to cause the bottom structure to "roll out" or flex outwardly at the juncture of the concavity and the surrounding heel, whereby the concavity becomes shallower and the radial dimension of the heel is altered. This, in turn, causes an increase in the volume enclosed by the bottle and a corresponding lowering of the level of liquid contained. Bottle engineering and design of the champagne bottom is required to control these tendencies.
Various expedients intended to alleviate these conditions have been proposed heretofore. Among them are the bottom structures disclosed in U.S. Pat. No. 3,468,443. The wall of each of these bottom structures is shown to be of a uniform thickness no greater than that of the sidewall. The portion of the wall which defines the central concavity is described as a "web." To rigidify this web a plurality of external ribs interrupt the outer surface of the concavity and extend outwardly therefrom. The ribs are distributed in a symmetrical array, each rib extending longitudinally in the direction of the heel from an inner portion of the concavity. Even with the rigidity provided by the ribs, some degree of eversion or flexure is expected, because a further, central depression is necessary to ensure that the center of the web will remain spaced from a flat supporting surface.
U.S. Pat. No. 3,598,270 teaches a plastic container suitable for carbonated beverages. The bottom of the container comprises portions conformed to meridian elements of a hemisphere with a downward pole and a plurality of hollow legs terminating in feet in a plane below the pole of the hemisphere. Each leg is defined by sidewalls diverging upwardly and outwardly relative to the polar position.
U.S. Pat. No. 3,871,541 teaches a bottom structure for plastic containers wherein an integral reinforcing rim is provided to expose the bottom dome to compressive stresses only, to arrest tensile and bending stresses at the base of the dome and to cut excessive material from the dome wherein the least amount of plastic material is used.
As mentioned above, one suitable bottom shape is a simple, outwardly hemispherical shape, but when this shape is utilized for plastic containers, a hemispherical shape requires a separately applied, outer peripheral support to make the bottle stand upright. A less expensive but more practical shape for plastic containers results from the inversion of the outwardly hemispherical shape to the inwardly concave or "champagne bottom" shape. The transition region located at the juncture of the cylindrical bottle side wall with the inverted, concave bottom forms a seating ring upon which the bottle is supported in an upright position. Much effort has been devoted to the design of inverted, concave bottoms of this type, and many different methods and many different molds have been developed.
It has been proposed that an initial outwardly convex bottom be blown which is then inverted to form a final upwardly concave bottom. Methods and apparatus proposed either (1) require the utilization of a separate inversion mold and reheating of the initial bottom or (2) simply push a convex die against the outwardly convex bottom. Neither technique has solved the problems inherent in the requirements of sharp curvatures in the transition zone and of adequate material thickness at the seating ring.
As a result, the prior art has not yet evolved a suitable bottom shape for forming a concave, pressure-resistant bottom for a thermoplastic container of light weight capable of retaining fluids under pressure, having a bottom highly resistant to deformation.